While indirect methods of producing molten iron (pig iron) from iron oxide material, e.g. iron ore, have been utilized for generations, in recent decades considerable emphasis has been placed on the direct production of molten iron from iron oxide.
The indirect approach is or can be differentiated from the direct approach in that the indirect approach requires numerous treatments before reaching the stage of molten iron. Generally these stages involve treatment with slags or the presence of slags.
In the direct method, the iron oxide ore or other oxidic iron-containing material can be directly reduced to elemental iron which can be smelted.
For example, in one direct method, the iron ore materials are treated with reducing gases and are thereby transformed into sponge iron. The latter is then smelted in a metallurgical vessel. In the smelting vessel a reaction is carried out between the metal and oxygen-containing gases whereby the oxygen from the gases reacts with carbon and carbon-containing substances, generally by blowing beneath the surface of the melt, thereby producing carbon monoxide and possibly, thermal energy.
The exothermically produced heat is partially utilized to smelt the sponge iron, and the waste gas from the process is utilized for the direct reduction of the ore. In general, the latter step can only be effected if all of the exhaust gas from the earlier step is reacted in a separable reactor with coal dust and steam.
In another prior art process, a combined smelting and gas generating reactor is used and is provided with an additional heat source. The fuel is reacted with oxygen to produce a reducing gas in this reactor and, within another compartment of the reactor, the reducing gas is passed in counterflow to the ore, the pre-reduced ore at the end of the reducing stage being then fed to the heated smelting and gas generating compartment in which the melt is formed and refined.
Another system for the direct production of pig iron utilizes two separate feed and reaction zones in smelting and gas generating reactors.
In a first zone, a carbon content above about 2% is maintained in the metal melt to which a carbon carrier is directly fed to this zone.
In a second zone, oxygen is reacted with a portion of the carbon contained in the melt to liberate heat and reducing gases. Carbon is fed into the system by an immersion lance which is plunged below the surface of the melt, to increase the carbonization of the iron bath and thus promote the smelting capacity and the formation of reducing gases.
In all of the afore-described systems, a principal phase in the operation is the production of a reducing gas which is utilized at least for the prereduction of the ore and even for the primary reduction thereof.
However, the production of reducing gases for these purposes requires excessive and complex control and measuring systems so that the reducing gas will have the correct composition.
Indeed, control is simplified by the separate production of the reducing gas, although this results in a significant increase in both the capital and operating costs.
In the face of these problems with the separate production of reducing gases and/or the complex regulation of the composition thereof, the assignee, in earlier work in this field, reflected in Luxembourg patent LU No. 82.227, for example, proposed a method for the direct production of molten pig iron in a single vessel whereby these disadvantages could be obviated. Specifically, the carbon carrier was blown directly by a neutral or reducing carrier gas and saturated with carbon. The iron oxide is deposited upon the bath surface and around the pile of iron oxide an oxygen blast is generated. The bath is agitated and maintained in flux by the continuous movement of gas upwardly through the bath from passages in the refractory bottom, i.e. blowing blocks which are gas permeable.
Surprisingly, it is found to be possible in this manner, i.e. by the top blowing of the carbon saturated bath with oxygen and the bottom bubbling of inert gas, to control the composition of the reducing gas formed above the surface of the melt and utilized for prereduction of the ore.
In practice it has been found that the gases produced at the surface of the melt can contain practically 100% carbon monoxide and thus have an extremely high reduction potential. The carbon monoxide concentration can be controlled by regulating the oxygen feed and the sparging of the bath with the inert gas such that the carbon monoxide content is increased with reduced bubbling of the sparging gas through the melt.
For example, the sparging of the bath with the inert gas can be reduced to 0-0.1 standard cubic meters of the inert gas per ton of melt per hour.
When it is desired to increase the thermal energy evolved at the surface of the melt, i.e. promote the reaction whereby carbon monoxide exothermically reacts with oxygen to produce carbon dioxide, the sparging gas flow can be increased and can reach amounts of 0.1-0.3 standard cubic meters of inert gas per ton of the melt per hour.
This additional heat facilitates smelting of the iron ore.
With this latter technique, however, the problem arises that a portion of the ore will become incorporated in the slag and will not be subjected to reduction.
This can be avoided by introducing the ore and the carbon by means of an immersion lance into the melt. Naturally this creates other problems, since such immersion lances are subject to wear, are expensive, and require space consuming and high maintenance manipulators and the like.
Bottom nozzles have also been provided to permit carbon and ore to be carried into the melt. Such bottom nozzles, as with immersion lances, are subject to a high degree of wear, are generally not long lived, are composed of expensive material and require frequent replacement and maintenance at high cost. The bottom nozzle generally must be supplied with gas continuously to prevent the penetration of metal from the bath into the passages of these nozzles. As a consequence, the consumption of the gases, which generally are not inexpensive, can be excessive.